In all weaving, an initially flat array of longitudinally extending warp threads is divided into at least two interspersed groups which are separated in opposite directions from the starting plane to define between the separated warp groups an elongated diamond shaped space, known as a "shed", through which the weft or filling is inserted, the direction of separation of the warp groups being reversed in a given order after each such weft by means of a harness motion with the result that the warp threads are entwined in sinuous fashion around successive filling threads to form the woven fabric. Traditionally, the weft is carried in coiled form upon a bobbin held within a shuttle, and as the weaving progresses, the shuttle is propelled alternatively back and forth through the shed on the upper surface of a beam-like lay which carried a comb-like reed projecting upwardly therefrom and rocks back and forth to press or "beat up" each new weft by means of the reed against the working end or "fell" of the fabric being woven. In the traditional loom, bobbin propulsion was accomplished by means of so-called picker sticks mounted on the loom adjacent opposite side edges of the warp for pivotal movement about their lower ends and driven to alternately impact their upper ends against the shuttle. Obviously, this conventional design was subject to inherent limitations as to achievable shuttle speed and was, moreover, accompanied by substantial disadvantages; namely, deafening operating noise as well as risk of breakage of picker sticks or other damage to equipment and of danger to operating personnel when, as occasionally happened, the shuttle escaped its containment and became an uncontrolled projectile. In order to overcome these inherent problems in bobbin type weaving, the prior art has explored various alternatives, and in the past decade or so, increasing attention has been directed to the possibility of impelling the weft thread through the shed by means of a jet of fluid. Jets of water have been found to be a relatively manageable projection medium, but water is a possible cause of corrosion and limits the choice of yarn material; thus there are significant advantages in the use of a gaseous fluid. While gases other than air can in theory serve equally well, cost considerations dictate the choice of air as the only practical gaseous propelling medium; consequently, this mode of weaving will hereinafter be referred to for convenience as "air weft insertion", although the use instead of other gases is, in principle, intended to be included.
In general, air projection techniques that have been used in past air weft insertion systems fall into two basic categories. In one type, the weft end is initially projected by means of a pressurized air from a nozzle situated outside and adjacent one side of the warp shed which serves to initially accelerate the weft end and starts its travel through the shed. The propulsion forces of existing nozzles is severely limited in terms of the attainable length of projection of the weft end and hence, in this type, a plurality of "booster" or supplemental jet nozzles is provided at spaced intervals through the shed, such nozzles being inserted within and removed in various ways from the shed interior via the clearance between the warp yarns. The aggregate of the propulsion forces of this multi-stage sequence of nozzles can be sufficient to convey the weft thread across the full width of the loom.
While this approach has proved generally feasible in practice, it too is faced with definite disadvantages, viz, the requirement for carefully controlled timing of the sequence of nozzle action plus excessive consumption of compressed air and thus poor economic efficiency.
In order to avoid the need for booster nozzles disposed at intervals through the shed, an alternative approach has been developed in a second type which utilizes a single exterior insertion nozzle in conjunction with a weft guidance "tube" situated within the shed. Since during weaving, the groups of warp threads must shift up and down past one another, the presence of any continuous body within the shed during shedding is out of the question. Therefore, an "interrupted" weft guidance tube is used, taking the form of a plurality of generally annular segments, each shaped to sufficiently narrow thickness in its axial dimension as to pass between adjacent wrap threads arranged in an axially aligned position so as to constitute together a lengthwise interrupted tubular member extending substantially the entirety of the shed width. Each annular segment has a slot-like exit opening at a point on its periphery to allow lateral egress of the inserted weft thread when the guidance tube is withdrawn below the shed. When the weft thread is projected by the exterior nozzle into one end of this interrupted guidance tube, the projection force imparted to the thread by the nozzle appears to be substantially enhanced so that the distance the weft thread is propelled by this force can be significantly increased compared to the nozzle alone.
The reasons why the interrupted guidance tube extends the projection force of the nozzle are not totally understood at present. The adjacent segments of this tube are separated by clearance spaces which are sufficient to permit pressurized air delivered into one end of the tube to disperse to the outside atmosphere while the interior edges of the bore of the segments should present considerable frictional resistance to movement of an air jet therethrough; from this standpoint the effect of such a tube might be expected to be negative. On the other hand, ambient air could be entrained from the ambient atmosphere into the interior of the tube through the same intersegment spaces with the possible effect of augmenting the propelling forces. In any event, it is established that the addition of a weft guidance tube generally as described above substantially increases the distance a weft thread can be projected with a jet of compressed air emitted from a nozzle and is virtually indispensable if the weft thread is to be effectively projected across the width of any practical size loom, say 48 inch or more in width.
Obviously, unless the leading end of the weft strand is projected entirely across the width of the loom with at least a short leading end portion protruding outside the opposite side of the shed, then there will result a defect in the fabric unless such nonarrival of the strand is detected virtually instantaneously and the weaving operation immediately terminated so as to allow the misdirected weft end to be corrected before the weaving operation is re-commenced. In order to assist the leading weft strand end in reaching the opposite shed side, it is fairly common practice in the weft insertion art to equip the loom with a means disposed adjacent the opposite side of the shed for attracting the leading strand end therein and, typically, such means takes the form of vacuum or suction tube having its opening facing the opposite shed side so as to aspirate the strand end therein. This kind of reception means is adapted readily to the detection of the arrival of the strand end by means of photoelectric or other strand sensing devices incorporated within the suction tube.
While these prior art arrangements have in principle proved advantageous, there is need in the art for further improvement, especially in terms of simplification of the necessary hardware, adaptation of the reception means to compensate for the varying position of the weft strand relative to the reed of the loom between the weft insertion and beat up positions of the reed, and enhancement of the durability and reliability of activation of the loom stop motion in the event of nonarrival of the weft strand.